1. Field of the Invention
The present invention is related to the field of radiation detectors. More specifically, the present invention is related to a radiation detector which is primarily sensitive to epithermal neutrons.
2. Discussion of the Relevant Art
Radiation detectors include devices capable of indicating the presence of free neutrons. Devices known in the art for indicating the presence of neutrons include helium-3 (He-3) proportional counters. He-3 counters typically are filled with gas under pressure, the gas being primarily composed of the isotope of helium having an atomic mass of 3 . Neutrons entering the He-3 counter typically react with the gas in the counter in such a way as to eventually cause the gas to ionize. When the gas in the counter ionizes a measurable change is generated in an electrical voltage applied to the gas through electrodes disposed in contact with the gas in the counter. The output of the counter generally consists of voltage pulses proportional in number to the number of neutrons detected by the counter.
The He-3 proportional counter is primarily sensitive to thermal neutrons. He-3 counters can also detect some epithermal neutrons, but at much lower efficiency than the detection of thermal neutrons. The significance of the types of neutrons and the relative efficiencies with which they are counted by the He-3 counter will be further explained.
Epithermal and thermal neutrons can be generated by the interaction of higher energy neutrons with atomic nuclei of other materials. For example, materials such as earth formations which can be penetrated by wellbore drilled into the earth, can be surveyed by a logging tool comprising a source of high energy neutrons and one or more neutron detectors. Higher energy neutrons can be generated by radioisotopic sources such as Americium-b 241 surrounded by a beryllium shield, or by electrically controllable pulse sources such as those described in, for example, "PDK-100" (wellbore logging tool), Atlas Wireline Services, Houston, Tex., 1992.
In such a wellbore logging tool, the high energy neutrons which are emitted from the source typically collide with the atomic nuclei of the materials forming the earth formations surrounding the wellbore. The neutrons will typically lose some of their energy content with each collision until their energy content reaches the epithermal level, and then the thermal level, whereupon these neutrons can be detected by the He-3 counter. Alternatively, thermal neutrons can be "captured" by atomic nuclei of certain materials which may be present in the earth formation, such as chlorine. Chlorine typically is present in earth formations in the form of sodium chloride dissolved in connate water. Connate water can be contained in voids, or pore spaces, which can form pan of some earth formations.
Certain properties of the earth formation can be determined by measuring the numbers of neutrons detected by the He-3 counter at a plurality of predetermined elapsed times from the generation of the high energy neutrons by the pulsed source. Alternatively, a measurement can be made of the number of neutrons detected by each one of a plurality of He-3 detectors positioned at spaced-apart locations from the source along the tool. A plurality of detectors is typically used when the neutron source is the Americium-241 type previously described, or is any other so-called "steady-state" source.
A limitation of He-3 counters for determining properties of earth formations is that He-3 counters, as previously stated, are primarily sensitive to thermal neutrons. Also as previously stated, thermal neutrons are subject to being "captured" by atomic nuclei of certain materials such as chlorine which may be present in the earth formation. Capture of thermal neutrons precludes their detection by the He-3 counter. The numbers of thermal neutrons captured by thermal absorbers such as chlorine in the earth formations is difficult to determine because the amount of chlorine in the earth formation is typically not known at the time the neutron tool is run in a wellbore. Unknown numbers of absorbed, and therefore undetected, thermal neutrons can cause erroneous determinations of the properties of the earth formations when the He-3 counter is used.
It is known in the art to determine the properties of the earth formation by measuring the numbers of epithermal neutrons present in the earth formation, the epithermal neutrons resulting from activation of the formation with high energy neutrons. As previously explained herein, epithermal neutrons have a higher energy content than thermal neutrons. Epithermal neutrons are much less susceptible to capture by chlorine or other materials which tend to capture thermal neutrons as a result of their higher energy. Epithermal neutron measurements are therefore less susceptible to error caused by undetermined chlorine concentration in some earth formations.
Using epithermal neutrons to determine properties of the earth formation requires a counter which is primarily sensitive to the epithermal neutrons. An epithermal neutron counter is described, for example, in U.S. Pat. No. 4,556,793 issued to Allen et al. The epithermal counter disclosed in the Allen patent includes the He-3 counter previously described herein, which is then enclosed first in a neutron moderating material, and then further enclosed in a "shielding" material which is capable of substantially preventing passage of thermal neutrons. The shielding material stops most of the thermal neutrons which may be present in the earth formation from entering the counter. Higher energy neutrons, which can pass through the shielding material, are reduced in energy by the moderating material. Reducing the energy of a neutron primarily means slowing down or reducing velocity. The neutrons are slowed to an energy level where they can be detected by the He-3 counter.
As previously explained, the He-3 counter is at least partially sensitive to epithermal neutrons, and the thermal neutrons are excluded from the counter by the shield, so the detector disclosed in the Allen patent responds primarily to epithermal neutrons.
A limitation to the use of the epithermal neutron detector described in the Allen patent is that the moderator changes the amount of time taken by the neutrons to reach an energy level at which their detection in the He-3 counter can occur, relative to the amount of time taken by the neutrons to reach the epithermal energy level only as a result of interaction with the earth formation. Measurements which are related to the amount of time taken for the high energy neutrons to slow down to epithermal energy levels as a result of interaction with the formation, are therefore distorted by using the detector disclosed in the Allen patent.
It is an object of the present invention to provide a neutron detector which is primarily sensitive to epithermal neutrons which does not significantly change the time distribution of the epithermal neutrons.